Many self-loading rifles with a rotating-lock bolt use direct gas impingement as their mechanism of operation. Gas is trapped from the barrel as the bullet moves past a gas port located above the rifle's front sight base. The gas rushes into the port and down a gas tube, located above the barrel, which runs from the front sight base into the rifle's upper receiver. Here, the gas tube protrudes into a “gas key” (bolt carrier key), which accepts the gas and funnels it into the bolt carrier.
The bolt and bolt carrier together effectively form a piston, which is caused to move as the cavity in the bolt carrier fills with high pressure gas. The bolt is locked into the barrel extension, so this expansion forces the bolt carrier backward a short distance in line with the stock of the rifle to first unlock the bolt. As the bolt carrier moves toward the butt of the gun, the bolt cam pin, riding in a slot on the bolt carrier, forces the bolt to turn and unlock from the barrel extension. Once the bolt is fully unlocked it begins its rearward movement along with the bolt carrier. The bolt's rearward motion extracts the empty cartridge case from the chamber, and as soon as the neck of the case clears the barrel extension, the bolt's spring-loaded ejector forces it out the ejection port in the side of the upper receiver. The bolt is much heavier than the projectile, and along with the recoil-spring pressure inside the stock buffer-tube performs the cartridge ejection function and chambers the following cartridge.
Behind the bolt carrier is a plastic or metal buffer which rests in line with a bolt return spring that pushes the bolt carrier back toward the chamber to return the bolt into battery. A groove machined into the upper receiver traps the cam pin and prevents it and the bolt from rotating into a closed position. The bolt's locking lugs then push a fresh round from the magazine which is guided by feed ramps into the chamber. As the bolt's locking lugs move past the barrel extension, the cam pin is allowed to twist into a pocket milled into the upper receiver. This twisting action follows the groove cut into the carrier and forces the bolt to twist and “lock” into the barrel's unique extension.
“Bolt bounce” is an undesirable phenomena associated with self-loading firearms, particularly fully automatic firearms. Specifically, when the bolt carrier comes forward and impacts the barrel extension, the potential exists for the bolt carrier to recoil slightly. This can have the undesirable effect of unlocking the bolt to a sufficient degree that the firearm is prevented from firing if the hammer falls again while the bolt is unlocked. Alternatively, if the bolt carrier is positioned excessively rearward when the hammer strikes the retracted firing pin during fully automatic fire, a misfire can occur because the bolt carrier absorbs energy from the hammer, thereby causing a too light strike by the hammer upon the firing pin.
Various solutions to the problem of bolt bounce exist, including the previously mentioned conventional buffered spring assemblies. Some of these have sliding weights inside the buffer portion, such as the buffer disclosed in U.S. Pat. No. 8,800,424, which is hereby incorporated by reference in its entirety. When the mass of the sliding weights is resonant with the rebound of the bolt carrier as it contacts the barrel extension, the forces will cancel each other, thereby keeping the bolt carrier against the barrel extension in the desired in battery position. However, some firearms cannot accommodate buffered spring assemblies in the stock, such as for pistol configurations. Furthermore, a conventional buffered spring assembly may be suitable for use with an unsuppressed fully automatic rifle using one kind of ammunition, but may have insufficient mass to adequately buffer the same rifle when a suppressor is used because of the greatly increased back pressure resulting from the suppressor, or when a different kind of ammunition is used. Overly energetic movement of the bolt carrier cannot only result in bolt bounce, but can also result in potentially excessive wear and tear on rifle components.
Therefore, a need exists for a new and improved firearm bolt assembly for a self-loading firearm that has a tunable bolt carrier mass that prevents bolt bounce. In this regard, the various embodiments of the present invention substantially fulfill at least some of these needs. In this respect, the firearm bolt assembly for a self-loading firearm according to the present invention substantially departs from the conventional concepts and designs of the prior art, and in doing so provides an apparatus primarily developed for the purpose of preventing bolt bounce.